Reusable delivery devices which can be reloaded with a new medicament container when a prior medicament container is emptied are known in the prior art. WO-2009/101005 describes an injector which is capable of an automatic adjustment of the plunger rod in relation to a plunger of a medicament cartridge. That injector comprises a dose setting mechanism capable of setting and resetting a dose of medicament and a drive nut connected to said dose setting mechanism. Said drive nut is rotatably coupled to the plunger rod by means of internal threads in an opening in the drive nut and external threads on the plunger rod. The injector further comprises a locking nut having an opening through which the plunger rod passes. The locking nut and the plunger rod are shaped such that an axial movement of the plunger rod relative to the locking nut is allowed while preventing a relative rotational movement between the plunger rod and the locking nut. When the injector is activated the drive nut is rotated and since the plunger rod is prevented from rotational movement by the locking nut the result is an axial movement of the plunger rod towards the plunger of the medicament container such that an amount of medicament is expelled through an injection needle of the delivery device.